The electrophysiological mechanisms of polymorphic ventricular tachycardia and fibrillation remain poorly understood. The surface manifestations as well as limited three-dimensional (3D) information obtained using arrhythmias. In the proposed study, we will utilize a new experimental method (transillumination) that should significantly enhance our ability to study 3D reentrant activity via visualization of its organizing center-filament. Using transillumination in combination with conventional optical mapping we will test the following major hypotheses: 1. In 3D ventricular myocardium, the filament tends to align parallel to myocardial fibers. 3. Sustained polymorphic ventricular tachycardia and fibrillation are maintained by stable filaments concealed in the depth of myocardial wall. Hypotheses 1 and 2 are based on preliminary computer simulations of 3D myocardial wall with realistic fiber geometry. Hypotheses 3 is supported by electrically induced tachycardia or fibrillation. The specific aims of our study are as follows: 1. To identify the mechanisms controlling spatial orientation and dynamics of the filament in computer models of ventricular wall with realistic fiber organization. 2. To study the evolution of the scroll-wave filament during sustained and non-sustained arrhythmias in isolated coronary-perfused preparations of sheep right and left ventricles using a transillumination technique. 3. To determine the role of stable filaments in the maintenance in the maintenance mechanisms of complex reentrant ventricular arrhythmias. The ultimate goal of this study is to assess the possibility of terminating those arrhythmias by controlling the evolution of the scroll wave filament.